Precision Measurement



Precision Measurement

Description

Precision measurement is a key engineering and design skill. Familiarity with measuring units and systems and experience with using precision measuring tools are essential CAD skills. Design cannot begin without an understanding of the components that will be used in the design. The dimensions of these components are vital to working drawings. For this reason, it is important that all dimensions be measured and recorded correctly and accurately.

Terms, Concepts, and Definitions

Dial Calipers: A measuring tool that measures to up to a 1000th of an inch.

Ruler/Scale: A measuring tool generally accurate to 1/16th of an inch. They are available in Standard, Metric, Architectural and Engineering units.

Vernier Height Gauge: A measuring tool used to find height from a surface.

Micrometers: Used to measure small distances or angles through the use of finely pitched screw threads.

Inside Calipers: This tool is used to record measurements inside bores, slots and the spaces in between parts and components. This tool is not used to measure directly, rather a dimension is “Captured” by adjusting the separation between the caliper ends. This separation distance is then measured off the caliper using a micrometer or a dial caliper.

Outside Calipers: Work similar to inside calipers, only they are used to “capture” outside dimensions of parts, components and assemblies.

11.2.3 Materials/Supplies

Dial Calipers

Ruler/Scale

Vernier Height Gauge (optional) (Learn to read a Vernier scale click here)

Components and assemblies to be measured.

Example: GEARS-IDS Invention and Design System Components

Objectives

Develop the skills to use precision measurement tools to accurately measure project components.

Use online calculators to convert standard measurements to metric, and vise versa.

Prerequisites

An understanding of the Standard and Metric systems of measurement.

Basic Algebra skills

Content

One of the end products of participating in the design process is the creation of working drawings. In order to create these drawings, it is necessary to know the dimensions of the components that will be used in the project. There are many tools that can be used to measure components. Three common and useful measuring tools will be discussed in this lesson: the Ruler or Scale, Dial Calipers, and the Vernier Height Gauge.

The Ruler or Scale

(Reasonably Accurate up to 1/100th of an inch or 0.254 mm)

The ruler is a familiar instrument. This tool is used to measure straight-line distances to accuracies of 1/16th of an inch . Some precision rulers have divisions of 1/100th of an inch. These rulers are rare, and not widely used. Despite the versatility of rulers, they are not as accurate as dial calipers and they are not the best choice for every situation.

Dial Calipers

(Very accurate to 0.001” or 0.0254 mm)

Dial Calipers are another type of measuring tool. They are generally used to measure distances that are six inches or less. Despite their relatively wide range of measurement (0.0 – 6” or more), they are extremely precise. Most Dial Calipers are accurate to within 1/1000th of an inch. Dial Calipers are also very easy to use, and they are designed in such way that they minimize human error. The ability to measure distances so accurately along with ease of use makes dial calipers the most popular choice for field engineers.

Vernier Height Gauge

This tool is somewhat specialized in that it is designed specifically to measure height. They are on average as precise as dial calipers. They are called Vernier Height Gauges if they measurement value is read directly from markings on the scale and not from a dial or liquid crystal display.

Skill in using these measuring tools is an important component of CAD. Measurements are specified in either the Standard or the Metric system. In certain cases it may be necessary to convert between the two systems. In these cases, a conversion factor will be needed to convert from one system to another. The internet has dozens of conversion calculator sites. Here are a few of my favorites:







Example 1:

A student decides that the counter balance arm she is using in her trebuchet needs to be lengthened. The arm is approximately 4 inches long. She uses Standard dial calipers and finds that the length between the pivot and the point of attachement of the counter weight is 4.146 inches. However, her teacher has instructed her to use metric values. It is necessary for this student to convert 4.146 inches to centimeters. Using an online converter or a conversion table she finds that 1 inch equals 2.54 centimeters. se performs the following calculation:

4.146 x 2.54 = 10.53 cm

The student finds that the counterweight arm is 10.53 centimeters long. She can now begin to design this component in CAD.

Example 2: Designing a Robot

A student needs to know how many speed controllers she can fit on a specific surface inside her Robot chassis. Using dial calipers she measures one of the speed controllers to be 1.485 inches by 1.168 inches. The surface is 6 centimeters by 10 centimeters. She performs the following calculations:

Step 1: Conversions

[pic]= 2.362in

[pic]= 3.937in

She has found that the Standard dimensions of the surface are 2.362in by 3.937in. She must now find the optimal configuration. To do this, she will use the measurements she took of the speed controller.

Step 2: Division

[pic]= 2.022[pic]2 This denotes that she will be able to fit 2 speed controllers in the smaller direction.

[pic]= 2.651 [pic]2 Note that the result of the calculation is greater than 2, but not greater than 3. Thus, she can only fit 2 speed controllers in the larger dimension.

Step 3: Multiplication

[pic]

By multiplying the number of speed controllers in one direction by the number in the other, she was able to find the total number of speed controllers she could fit on the surface.

It is important to note that as a result of her precision measurements and use of simple math, she was able to solve a problem quickly and accurately. Notice the calculation for the smaller dimension. If she had not used precise measurement, she could have easily found that she would not be able to fit 2 speed controllers in that dimension. Due to her ability to measure accurately with dial calipers, she was able to determine that she would be able to fit 2 speed controllers in the smaller dimension.

Activity

The GEARS-IDS Mechanical Bread Boarding Kits are a set of components that can be used to create machines. These kits are endlessly useful in engineering, as they allow for fast transition from ideas to working constructions.

Assign students to select 2-3 trebuchet components and to sketch these parts in two and three views. Have the students use precision measuring tools to add dimensions to those sketches. Two of the drawings should be dimensioned in Metric units, and two should be dimensioned in Standard units. Any precision measuring tool can be used, but accuracy is the goal. Also, these measurements will be used in a later lesson to draw these parts in a CAD program, and those parts will ultimately be used to illustrate engineering principles.

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